linux-kernel - Re: [PATCH] ACPI: Drop rcu usage for MMIO mappings

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Date:   Thu, 7 May 2020 14:20:30 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Cc:     stable <stable@...r.kernel.org>, Len Brown <lenb@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Ira Weiny <ira.weiny@...el.com>,
        James Morse <james.morse@....com>,
        Erik Kaneda <erik.kaneda@...el.com>,
        Myron Stowe <myron.stowe@...hat.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH] ACPI: Drop rcu usage for MMIO mappings

On Thu, May 7, 2020 at 9:43 AM Rafael J. Wysocki
<rafael.j.wysocki@...el.com> wrote:
>
> On 5/7/2020 1:39 AM, Dan Williams wrote:
> > Recently a performance problem was reported for a process invoking a
> > non-trival ASL program. The method call in this case ends up
> > repetitively triggering a call path like:
> >
> >      acpi_ex_store
> >      acpi_ex_store_object_to_node
> >      acpi_ex_write_data_to_field
> >      acpi_ex_insert_into_field
> >      acpi_ex_write_with_update_rule
> >      acpi_ex_field_datum_io
> >      acpi_ex_access_region
> >      acpi_ev_address_space_dispatch
> >      acpi_ex_system_memory_space_handler
> >      acpi_os_map_cleanup.part.14
> >      _synchronize_rcu_expedited.constprop.89
> >      schedule
> >
> > The end result of frequent synchronize_rcu_expedited() invocation is
> > tiny sub-millisecond spurts of execution where the scheduler freely
> > migrates this apparently sleepy task. The overhead of frequent scheduler
> > invocation multiplies the execution time by a factor of 2-3X.
> >
> > For example, performance improves from 16 minutes to 7 minutes for a
> > firmware update procedure across 24 devices.
> >
> > Perhaps the rcu usage was intended to allow for not taking a sleeping
> > lock in the acpi_os_{read,write}_memory() path which ostensibly could be
> > called from an APEI NMI error interrupt? Neither rcu_read_lock() nor
> > ioremap() are interrupt safe, so add a WARN_ONCE() to validate that rcu
> > was not serving as a mechanism to avoid direct calls to ioremap(). Even
> > the original implementation had a spin_lock_irqsave(), but that is not
> > NMI safe.
> >
> > APEI itself already has some concept of avoiding ioremap() from
> > interrupt context (see erst_exec_move_data()), if the new warning
> > triggers it means that APEI either needs more instrumentation like that
> > to pre-emptively fail, or more infrastructure to arrange for pre-mapping
> > the resources it needs in NMI context.
> >
> > Cc: <stable@...r.kernel.org>
> > Fixes: 620242ae8c3d ("ACPI: Maintain a list of ACPI memory mapped I/O remappings")
> > Cc: Len Brown <lenb@...nel.org>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Cc: Ira Weiny <ira.weiny@...el.com>
> > Cc: James Morse <james.morse@....com>
> > Cc: Erik Kaneda <erik.kaneda@...el.com>
> > Cc: Myron Stowe <myron.stowe@...hat.com>
> > Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> > Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > Signed-off-by: Dan Williams <dan.j.williams@...el.com>
>
> linux-acpi is kind of relevant for this too, so please CC it.

Whoops, my bad. Will resend with some of Andy's cleanup suggestions.